Please note that an EU National does not need official "Residence" in Spain.
They only need to register in person at the Oficina de Extranjeros in their province of residence or at designated Police stations if staying in spain for more than 3 Months.
You will be issued with a green A4 printed sheet called a “Certificado de Extranjeros” stating your name, address, nationality, NIE number (Número de Identificación Extranjeros) and date of registration.
The link below explains how you can apply for RESIDENCE (not registration) in Spain and what documents and requirements you need to meet.From 28 March 2007, the Royal Decree 240/07 stated that all EU citizens planning to live in Spain for more than three months should register in person at the Oficina de Extranjeros in their province of residence or at designated Police stations. However, you will no longer be issued with a residence card with a printed photograph… as these ceased to be issued some time ago. Instead you will be issued with a green A4 printed sheet called a “Certificado de Extranjeros” stating your name, address, nationality, NIE number (Número de Identificación Extranjeros) and date of registration.
https://www.gov.uk/residency-requirements-in-spain